Certain types of membranes have been used for many years to separate selected gases from air and other gas mixtures. Composite hollow fibers, employing organic polymer membranes, may have separation factors that favor the permeation of oxygen over nitrogen by a factor of ten or less. Processes employing such membranes have been devised for the production of oxygen and particularly nitrogen from ambient air.
An entirely different type of membrane can be made from certain inorganic oxides, typified by calcium or yttrium stabilized zirconia and analogous oxides with the fluorite structure. At elevated temperatures, these materials contain mobile oxygen-ion vacancies. When an electric field is applied, these materials will transport oxygen, and only oxygen, and thus can act as a membrane with an infinite selectivity for oxygen. These membranes are thus very attractive for use in new air separation processes.
Although the potential for these materials as gas-separation membranes is great, there are certain problems in their use. The most obvious problem is that all known materials exhibit appreciable oxygen-ion conductivity only at elevated temperatures. In general they must be operated well above 500.degree. C. Much research has been done to find materials that work at lower temperatures, but this limitation remains.
Electrically driven oxide membranes require conducting electrodes on both surfaces for the application of the electric field. These electrodes should, preferably, be porous or otherwise permeable to air and oxygen. Materials such as ceramic lanthanum strontium cobaltite fulfill these requirements. The reaction of oxygen has been shown to occur in the region where all three phases, gas-electrode-electrolyte, converge: EQU ( 1/2)O.sub.2 (g)+2'.revreaction.O" (1) EQU O"+[V.sub.O" ].sub.electrolytes =nil
The oxygen ions annihilate oxygen-ion vacancies [V.sub.O" ] which are highly mobile in the electrolyte. At the cathode, two electrons must be supplied for each oxygen ion created, or four electrons for each molecule of oxygen gas that is ionized. Thus 4 Faradays or 386 million Coulombs of charge must be supplied for each kmole of oxygen transported. The required electrical current is: EQU I=386.times.10.sup.6 .times.Q (2)
where: Q is the oxygen flux in kmols s.sup.-1.
The theoretical minimum voltage required is given by the Nernst equation: ##EQU1## where:
R is the gas constant=8.31.times.10.sup.3 J kmol.sup.-1 K.sup.-1
T is the Temperature, .degree. K.
F is Faradays constant=9.65.times.10.sup.7 C kmol.sup.-1
p.sub.1 is the partial pressure of O.sub.2 on the cathode side
p.sub.2 is the partial pressure of O.sub.2 on the anode side
Equation (3) is hereinafter referred to as the Nernst Equation. The oxygen partial pressure, p=Y.sub.o .times.P, is the product of the oxygen mole fraction and the total pressure.
The power required is the product of the current and the voltage. It is apparent that the power is necessarily high when large quantities of oxygen are to be transported. For this reason, the electrically driven processes are less attractive for the bulk separation of oxygen from air, except for small specialized applications.
A process using electrically driven oxide membranes is much more attractive for the removal of small quantities of oxygen from nitrogen, argon or other gas streams. In this case, the power needed depends on the partial pressure of oxygen that can be tolerated in the product stream, on the cathode side of the membrane. Since only oxygen is transported, the anode side is usually pure oxygen. The minimum voltage must be greater than that given by the Nernst equation for these conditions. Unfortunately, even this minimum required power is too large for many commercial applications.
It has been a problem to find a practical means to reduce the power required for removing oxygen from a gas stream by electrically driven permeation through a solid-electrolyte membrane. Although research on electrolytic oxygen-ion conductors has been carried on for many years, these processes seldom have been used commercially for gas separation or purification. One reason for this is the large electrical power required by these processes per unit amount of O.sub.2 removal. Because of the infinite selectivity exhibited by solid-electrolyte membranes, most of the interest in these materials has been for producing small quantities of pure O.sub.2 for specialized applications.
Recent advancements in the state of the art of air separation using inorganic oxide membranes have been presented in the technical literature. For example, in a 1977 paper entitled "Ionically Conducting Solid-State Membranes" in the journal Advances in Electrochemistry and Electrochem. Engrg., R. A. Huggins provided an early review article on all types of solid-state ionic conductors, including cubic stabilized zirconia and other oxides of the fluorite structure.
In their 1992 paper entitled "Separation of Oxygen by Using Zirconia Solid Membranes", appearing in Gas Separation Purification, Vol. 6, No. 4, at pages 201-205, D. J. Clark, R. W. Losey and J. W. Suitor describe the production of O.sub.2 for special applications such as space travel. While multicell stacks are described, there is no mention of "taging" as provided by the present invention.
Turning to the patent literature, U.S. Pat. No. 4,725,346 to Joshi describes a device and assembly for producing oxygen, using oxygen-conducting metal oxide electrolytes. Subsequently, in U.S. Pat. No. 5,021,137, Joshi et. al. describe a cell based on doped cerium oxide with lanthanum strontium cobaltite electrodes.
In U.S. Pat. No. 5,045,169, Feduska et. al. disclose various device configurations wherein several electrochemical cells are connected so that they are electrically in series, thus raising the overall voltage to a more practical value. The devices described are for the production of oxygen, not for the removal of oxygen from inert gas streams. Nor does the patent describe the use of multiple cells connected in two or more stages with respect to the gaseous stream being treated, which is the subject matter of the present disclosure.
U.S. Pat. No. 5,035,726 to Chen discloses the use of electrically driven solid-electrolyte membranes for the removal of low levels of oxygen from crude argon streams. He estimates the electrical power needed for several examples of multistage processes. The voltage is constant for the early stages of the examples cited. The potential benefits of staged processes in reducing the power are thus not fully realized.
In U.S. Pat. No. 5,035,727, also to Chen, advantage is taken of the high-temperatures available from the exhaust of an externally-fired gas turbine to produce oxygen by permeation through a solid-electrolyte membrane.